In the medical/nursing care field, an electrically powered bed that is capable of inclining, raising, and lowering a back bottom or a knee bottom of the bed is widely used in order to reduce the burden of eating, sleeping, and the like on a patient or a person requiring care. In such an electrically powered bed, an electrically powered feed-screw type linear actuator is used because such a linear actuator is compact and is capable of obtaining a large drive force. For example, in Patent Document 1, a linear actuator is described, which is configured to cause a piston to extend and contract using a ball screw mechanism. In the linear actuator of Patent Document 1, the rotation of an electric motor is decreased and transmitted to a drive shaft by a worm and a worm wheel. The rotation of the drive shaft is converted into linear motion by the ball screw mechanism, which causes the piston to extend or contract. The linear actuator of Patent Document 1, which is used in the reclining of the back bottom of a bed, is connected to a link mechanism of the bed. When the piston of the actuator extends, the link mechanism expands, and the back bottom of the bed rises up to be in an inclined state (refer to FIG. 1(b)). Further, when the piston contracts, the link mechanism folds away, the back bottom is lowered into a horizontal position, and the bed shifts to a flat state (refer to FIG. 1(a)).